Fuel tanks of the type which are installed in the body framework or cavity of vehicles, and particularly, those which are made for aircraft installation, have been made for many years by a labor-intensive, inside-to-outside layup process. According to one of these prior art processes, a plaster/wood-wool mixture is formulated and applied to a mold fixture having the approximate configuration of the body framework cavity into which the fuel tank is to be installed. The fixture, however, is made undersized by an amount equal to the wall thickness of the finished tank. The plaster is cured for approximately 48 hours and removed from the fixture and the resultant male plaster form is used for lay-up of the fuel tank structure. Accordingly, because it is a male form, the first ply of material applied to the plaster form will constitute the interior facing of the finished tank. Therefore, to eliminate any possible discontinuities in the composite structure which may compromise the leak-proof integrity of the tank, the plaster form must be made as smooth as possible. Consequently, the plaster is hand-sanded to a smooth finish and then the various fittings which are required for the finished tank are mounted thereon. Finally, the materials forming the composite of the tank wall structure are applied to the plaster form and cured into an integral structure. Upon completion of the cure, the plaster form must be removed from the inside of the tank. This is accomplished by soaking the plaster in hot water, breaking it up into small pieces, and washing it out of the tank through one or more of the access fittings provided in the tank wall. Obviously, it is extremely important that all of the plaster is removed from the interior of the tank or contamination of the fuel will result.
Exemplary of the prior art pertaining to fuel tank manufacturing and the use of male forms are U.S. patents to Scharenberg (U.S. Pat. No. 2,394,492); Bell (U.S. Pat. No. 2,394,423); Noyes et al. (U.S. Pat. No. 2,890,489; and Bailey (U.S. Pat. No. 2,558,807).
While the described prior art process and the techniques of the above-referenced prior art patents have been used successfully for many years, the primary disadvantage centers upon the use of the male form. Because the male form is destroyed in the process of removing it from the interior of the tank, a new form is required for each and every tank made. Thus, the costs attendant to the making of these type of complex-shaped fuel tanks have risen dramatically because of the labor intensive nature of a process requiring hand-made male forms.
Furthermore, and because the composite forming the tank wall structure is laid up on the male form, it must be applied to a plurality of recesses which form the complex shape of the plaster mold. Irrespective of whether the composite is laid up by spraying or by hand application techniques, the recesses are critical areas of the tank structure and careful attention must be paid when applying the composite materials to these areas.
In view of the aforementioned disadvantages of the prior art tank manufacturing process which employs the use of male forms, reverse building processes using reverse build (female) molds have become more common. The reverse building process permits one to build more complex-shaped fuel tanks much more readily and easily than the aforementioned process. This reverse building process also eliminates some common installation problems when mounting the fuel tank in the complex-shaped vehicle cavity. For example, fittings can be more accurately located in the fuel tanks than was previously done when male forms were used.
Heretofore, the reverse building process for the manufacture of complex-shaped, fabric-reinforced, elastomeric fuel tanks has been typically accomplished by the method provided in Robinson et al. U.S. Pat. No. 4,434,021. Such a process uses complementary female mold half sections which, when joined together about their mating peripheral edges, exactly duplicate the geometry of the body framework cavity of the vehicle into which they are to be mounted. After applying a ply of release material to the mold sections and installing tank fittings where needed, plies of urethane elastomer may be spray-coated into the molds and nylon fabric reinforcement may be applied in two steps. The nylon fabric reinforcement may then be spray coated with another layer of urethane elastomer and a barrier system is then applied. The composite structure is again spray coated with more urethane elastomer to complete the composite which forms the tank structure. Upon completion of forming of the composite, the tank mold halves are joined by clamping them together, and a splice is completed from the inside of the tank by laying in reinforcement fabric at the butt joined sections of the composite and sealing it with urethane elastomer. Finally, the resulting fuel tank is allowed to cure into an integral structure and is removed from the molds to be positioned into the vehicle body cavity.
Such fuel tanks built in reverse order (outside-to-inside) may be advantageously lighter in weight than those built using male forms. This is because light weight materials which are known to be more conformal and flexible than those materials typically used in the process utilizing the male form, can be used in forming the complex-shaped fuel tanks. A light weight fuel tank is highly desirable for many vehicles such as aircraft, where weight continually presents problems in the design and use of such vehicles.
However, there remains disadvantages associated with the use of these light weight materials, such as the nylon fabrics disclosed above, in the production of the complex-shaped fuel tanks. For example, unlike the materials which may be used in forming complex-shaped fuel tanks using the male forms, the light weight materials are not self-sealing, meaning that upon being penetrated by a object such as a bullet from an anti-aircraft gun in the case of a fighter aircraft, the fuel tank may begin to leak fuel through the hole left by the bullet in the material, instead of sealing or, at the least, reducing the size of the hole through which fuel may flow out of the tank. While the light weight fuel tanks may include tear-resistant fabrics, meaning that upon penetration of the object, the hole left by the object should not increase significantly in size due to tearing of the fabric, this fabric does not prevent fuel from leaking from the fuel tank which could ultimately create a potentially dangerous or otherwise hazardous situation to the operator of the vehicle, such as the pilot of a fighter aircraft.
There are also disadvantages to the particular reverse build process disclosed in Robinson et al. U.S. Pat. No. 4,434,021. For instance, because of the way in which the fuel tanks are manufactured, the materials used are not autoclave curable. As noted in Robinson et al. U.S. Pat. No. 4,434,021, the two halves of the tank structure are joined together while still within the female mold sections to form the complete tank structure, the butt joined ends of the halves being spliced together and sealed. Only after the butt ends have been joined and the entire tank air cured (for at least approximately 24 hours) are the female mold sections removed. By that time, the tank structure has become one integral unit.
In order to cure materials in an autoclave or other heating apparatus where female molds are used, it is necessary to provide pressure to the inside of the molded part. However, because the female mold sections are joined prior to curing, as in Robinson et al. U.S. Pat. No. 4,434,021, it is extremely and prohibitively expensive, if not impossible, to provide the necessary pressure to the inside of the completed integral tank. In other words, because of the manufacturer's inability to provide sufficient pressure on the inside of the composite tank structure, autoclave or oven curing is not practical for the materials employed in the complex-shaped fuel tanks which are built using the reverse build process.
Accordingly, the light weight materials and, for that matter, any materials employed, must be cured at ambient (or slightly higher) temperature and at normal air pressure over a significantly longer period of time than would materials used with the male forms. While it should be clear that there are many advantages to using a reverse build process and light weight materials in that process, disadvantages associated with the use of these materials (not self-sealing) and the process as heretofore known (incapable of autoclave curing) reduce the desirability for the product and add significantly to the amount of time required to manufacture the fuel tank which, in turn, adds to the overall cost of the fuel tank.
Thus, the need exists for a manufacturing process that will permit at least a portion of a highly conformal, fabric-reinforced, elastomeric fuel tank to be built using a reverse build (female) mold and self-sealing, autoclave curable materials. The fuel tank can then be completed by joining the self-sealing portion of the tank with a light weight portion made from different materials.
Significant manufacturing costs are further experienced where tank fittings must be prefabricated or premolded prior to installation within the female molds. Such tank fittings typically included fitting metals which are bonded to a separately manufactured flange material using additional molding equipment. The flange portion of the fitting, once bonded to the fitting metals as by a clamping process or the like, is then placed into the female mold, and the tank is then built on top of it. Because each fitting is manufactured separately from the tank wall itself, it will be appreciated that the thickness of the tank will practically double at each fitting location, due to the increased number of layers of material constituting the flange material.
Moreover, because the fitting metals have to remain free from any of the other materials used to manufacture the fuel tank (to ensure the fitting would be suitable for use in the cavity of the vehicle framework), it was heretofore required that the fitting metals be covered by tape or the like to prevent such other materials from contaminating the fittings. Such the accurate location of the fittings is one of the primary objects of using the reverse build process, as noted in Robinson et al. U.S. Pat. No. 4,434,021, it is seen as highly desirable to maintain the ability to accurately locate and install the tank fittings, while reducing the cost of fabricating the same.